The proposed work addresses the need for information on two congruent problems which require resolution. These are: a) the risk assessment of the absorbed radiofrequency (RF) energy that might result from magnetic resonance (MR) diagnostic procedures in the human and b) the adequacy of the 0.4 W/kg protection guide recommended by the American National Standards Institute (ANSI) for RF exposure. In this context we propose to characterize the response of the rat neuroendocrine and thermoregulatory systems. Radioimmunoassays for ACTH, Beta-endorphin, prolactin, TSH, T4 corticosterone will be carried out on blood; ACTH and Beta-endorpin will be assayed on specific brain loci to elucidate the important interrelationships of the central and peripheral hormone levels in the response of the endocrine and thermoregulatory systems to varying energy absorption levels from RF and 2 T magnetic fields exposure. In contrast to conventional procedures which permit assay only of terminal samples for neuroendocrine interrelationships (i.e., on blood obtained immediately after decapitation and exanguination of the rat), the present proposed work will utilize a carotid-cannulated preparation to avoid undesirable exogenous or endogenous effects related to rat anticipatory and handling (stress) reaction. The neuroendocrine response will be characterized for RF exposures at, above and below the ANSI standard and FDA recommendation for MR/RF specific absorpton rate (SAR) of 0.4 W/kg and 2 T magnetic (MR) fields. Neuroendocrine responses will be correlated with colonic and brain temperature changes. The special rat model will be used to assess the relative sensitivity of deviation from normal which could be relevant and of concern in the clinical setting. These include apprehension, obesity, hypertension, altered blood-brain barrier integrity or pregnancy.